Core Java - Interview Questions and Answers

51) Why is multiple inheritance not supported in java?

To reduce the complexity and simplify the language, multiple inheritance is not supported in java. Consider a scenario where A, B, and C are three classes. The C class inherits A and B classes. If A and B classes have the same method and you call it from child class object, there will be ambiguity to call the method of A or B class.

Since the compile-time errors are better than runtime errors, Java renders compile-time error if you inherit 2 classes. So whether you have the same method or different, there will be a compile time error.

 class A{ void msg(){System.out.println("Hello");} } class B{ void msg(){System.out.println("Welcome");} } class C extends A,B{//suppose if it were    Public Static void main(String args[]){    C obj=new C();    obj.msg();//Now which msg() method would be invoked? } } 

Test it Now

  Compile Time Error

52) What is aggregation?

Aggregation can be defined as the relationship between two classes where the aggregate class contains a reference to the class it owns. Aggregation is best described as a has-a relationship. For example, The aggregate class Employee having various fields such as age, name, and salary also contains an object of Address class having various fields such as Address-Line 1, City, State, and pin-code. In other words, we can say that Employee (class) has an object of Address class. Consider the following example.

Address.java

 public class Address { String city,state,country;  public Address(String city, String state, String country) { 	this.city = city; 	this.state = state; 	this.country = country; }  } 

Employee.java

  public class Emp { int id; String name; Address address;  public Emp(int id, String name,Address address) { 	this.id = id; 	this.name = name; 	this.address=address; }  void display(){ System.out.println(id+" "+name); System.out.println(address.city+" "+address.state+" "+address.country); }  public static void main(String[] args) { Address address1=new Address("gzb","UP","india"); Address address2=new Address("gno","UP","india");  Emp e=new Emp(111,"varun",address1); Emp e2=new Emp(112,"arun",address2); 	 e.display(); e2.display(); 	 } } 

Output

 111 varun gzb UP india 112 arun gno UP india

53) What is composition?

Holding the reference of a class within some other class is known as composition. When an object contains the other object, if the contained object cannot exist without the existence of container object, then it is called composition. In other words, we can say that composition is the particular case of aggregation which represents a stronger relationship between two objects. Example: A class contains students. A student cannot exist without a class. There exists composition between class and students.

54) What is the difference between aggregation and composition?

Aggregation represents the weak relationship whereas composition represents the strong relationship. For example, the bike has an indicator (aggregation), but the bike has an engine (composition).

55) Why does Java not support pointers?

The pointer is a variable that refers to the memory address. They are not used in Java because they are unsafe(unsecured) and complex to understand.

56) What is super in java?

The super keyword in Java is a reference variable that is used to refer to the immediate parent class object. Whenever you create the instance of the subclass, an instance of the parent class is created implicitly which is referred by super reference variable. The super() is called in the class constructor implicitly by the compiler if there is no super or this.

 class Animal{ Animal(){System.out.println("animal is created");} } class Dog extends Animal{ Dog(){ System.out.println("dog is created"); } } class TestSuper4{ public static void main(String args[]){ Dog d=new Dog(); } } 

Test it Now

Output:

 animal is created dog is created

57) How can constructor chaining be done by using the super keyword?

 class Person {     String name,address;      int age;     public Person(int age, String name, String address)     {         this.age = age;         this.name = name;         this.address = address;     } } class Employee extends Person  {     float salary;     public Employee(int age, String name, String address, float salary)     {         super(age,name,address);         this.salary = salary;     } } public class Test  {     public static void main (String args[])     {         Employee e = new Employee(22, "Mukesh", "Delhi", 90000);         System.out.println("Name: "+e.name+" Salary: "+e.salary+" Age: "+e.age+" Address: "+e.address);     } } 

Output

 Name: Mukesh Salary: 90000.0 Age: 22 Address: Delhi

58) What are the main uses of the super keyword?

There are the following uses of super keyword.

super can be used to refer to the immediate parent class instance variable.
super can be used to invoke the immediate parent class method.
super() can be used to invoke immediate parent class constructor.

59) What are the differences between this and super keyword?

There are the following differences between this and super keyword.

The super keyword always points to the parent class contexts whereas this keyword always points to the current class context.
The super keyword is primarily used for initializing the base class variables within the derived class constructor whereas this keyword primarily used to differentiate between local and instance variables when passed in the class constructor.
The super and this must be the first statement inside constructor otherwise the compiler will throw an error.

60) Can you use this() and super() both in a constructor?

No, because this() and super() must be the first statement in the class constructor.

Example:

 public class Test{ 	Test()      {          super();           this();          System.out.println("Test class object is created");      }      public static void main(String []args){      Test t = new Test();      } } 

Output:

 Test.java:5: error: call to this must be first statement in constructor

Interview :: Core Java